1. Field of the Invention
The present invention relates to devices for compressing/decompressing map data, and more particularly to devices for compressing/decompressing data which represents shapes of objects on a map in vector format.
2. Description of the Background Art
In map data, objects required for rendering a map (typically, roads, buildings, rivers, and green belts) are each generally represented in vector format by a set of element points for specifying the shape of the object. Conventionally, each object is represented by a string of sets of element points all of which are typically represented by normalized coordinate values. However, such map data is not suitable for distribution via communication or broadcasting because of its data size, therefore in recent years, various map data compression methods have been proposed. Hereinafter, an exemplary conventional map data compression method is described.
In the conventional compression method, element points of each object are represented by either normalized coordinate values or first-order differential values. Here, the “first-order differential value” refers to a differential value between a normalized coordinate value of a target element point and a normalized coordinate value of the one before the target element point. For example, in the case where the normalized coordinate value of the target element point is (X1, Y1) and the normalized coordinate value of the immediately preceding element point is (X0, Y0), the first-order differential value is represented by (X1−X0, Y1−Y0). Such a first-order differential value is employed to compress the size of data for specifying each element point (see, for example, European Patent Publication No. EP1134674 A1).
It is expected that the demand for map distribution is further increased in the future, and map data is used even with data terminal apparatuses which can be equipped with merely a small capacity storage device. However, conventional compression methods are unsatisfactory with respect to size of compressed data, and therefore disadvantageous in that they cannot be adapted to an increase in use of the map data.
Most of the data terminal apparatuses that receive map distribution are mobile apparatuses such as navigation apparatuses, and therefore acquire map data through a mobile communication network. However, in most cases, the billing system for mobile communication services is based on the amount of use, therefore the conventional compression methods are disadvantageous in that users' communication cost adds up considerably because of the unsatisfactory size of compressed data.